votes: make private favorites and upvotes a Gold-only option.

Make private favorites and upvotes a Gold-only account option.

Existing Members with private favorites enabled are allowed to keep it
enabled, as long as they don't disable it. If they disable it, then they
can't re-enable it again without upgrading to Gold first.

This is a Gold-only option to prevent uploaders from creating multiple
accounts to upvote their own posts. If private upvotes were allowed for
Members, then it would be too easy to use fake accounts and private
upvotes to upvote your own posts.

app/models/user.rb

@@ -103,6 +103,7 @@ class User < ApplicationRecord
   validates :per_page, inclusion: { in: (1..PostSets::Post::MAX_PER_PAGE) }
   validates :password, confirmation: true
   validates :comment_threshold, inclusion: { in: (-100..5) }
+  validate  :validate_enable_private_favorites, on: :update
   before_validation :normalize_blacklisted_tags
   before_create :promote_to_owner_if_first_user
   has_many :artist_versions, foreign_key: :updater_id
@@ -192,6 +193,14 @@ class User < ApplicationRecord
     end
   end
 
+  concerning :ValidationMethods do
+    def validate_enable_private_favorites
+      if enable_private_favorites_was == false && enable_private_favorites == true && !Pundit.policy!(self, self).can_enable_private_favorites?
+        errors.add(:base, "Can't enable privacy mode without a Gold account")
+      end
+    end
+  end
+
   concerning :AuthenticationMethods do
     def password=(new_password)
       @password = new_password